Toilet training

Ladiessss, please help. My son if fully potty trained in the day but at home he refuses to wear pants or trousers and he constantly plays/fiddles. Obviously this is normal and natural but what can I do to teach him not to do it when we’re all sat together because he’s constantly giving himself a hard on and he’ll be like mummy look and show me and do stuff to it 😭 he’s never seen any kind of behaviours like that so I know it’s a natural thing to experiment but yeah being a single mum of two boys I’m not sure how to approach this one 😭